Photonic Crystal Improves Output of Quantum Dot Films

The addition of a 2-D photonic crystal to the surface of LED structures boosts the output of these devices by freeing light that otherwise would be trapped inside. A team of scientists from the University of California, Santa Barbara, and from Mitsubishi Chemical Research and Innovation Center in Goleta, Calif., has found that a photonic crystal similarly increases light extraction from a film of colloidal quantum dots. The discovery may affect the development of biological tagging methods and of proposed devices incorporating quantum dots such as LEDs.

Frédéric S. Diana, a doctoral student in Pierre M. Petroff’s group at the university, explained that the trapping of guided modes is a significant problem in planar layered structures with refractive indices higher than 1.5 that incorporate light-emitting materials. Rather than leaving the structure in the intended direction, some of the light generated is trapped and absorbed or escapes in the wrong direction. In thin films of quantum dots, he said, guided modes account for at least 70 percent of the total radiated power, which usually is lost.

Like all planar layered structures with refractive indices higher than 1.5 that incorporate light-emitting materials, quantum dots in a thin film produce a significant portion of their light as guided modes, which can be seen here exiting through the edges of the glass slide substrate. Courtesy of Frédéric S. Diana.
As part of their investigation into the potential of quantum dots to replace traditional down-conversion phosphors in white LEDs, the scientists used imprint lithography and reactive ion etching to define and produce a 50-nm-deep triangular 2-D photonic crystal in a 1.75-μm-thick GaN planar cavity incorporating three InGaN quantum wells. They then coated the structure by drop-casting with a 100-nm-thick layer of CdSe/ZnS core/shell quantum dots emitting at 620 nm.

To measure the effect of the periodic structure on light extraction, they performed far-field photoluminescence and microphotoluminescence studies of the film under 325-nm excitation with a HeCd laser from Kimmon Electric US LP of Englewood, Colo. Images of the response were collected using a spectrometer and a 1024 × 256-pixel cooled CCD detector, both from Horiba Jobin Yvon Inc. of Edison, N.J. They found that the photonic crystal increased the external quantum efficiency of the film by at least 40 percent and acted to make the output more highly directional, Diana said.

Microphotoluminescence measurements of the quantum dot film reveal the characteristic pattern of extracted light induced by the photonic crystal. The incorporation of the structure resulted in an increase in the external quantum efficiency of at least 40 percent.
He explained that quantum dots are of interest as replacements for inorganic down-conversion phosphors in high-power white LEDs because the dots can be incorporated into films that display little or no scattering and low self-absorption. The performance of phosphors is hampered by their minimum grain size of at least 1 μm, which leads to their action as strong scattering centers, reducing the external efficiency and limiting control of the far-field emission pattern.

The challenge is that the traditional dome-shaped encapsulants used in LEDs may not be suitable and that fabrication of photonic crystals is not economically practical for use in commercial devices, although Diana suggested that imprint lithography may overcome this hurdle.

To improve light extraction from a film of CdSe/ZnS quantum dots, researchers etched a triangular 2-D photonic crystal in the GaN layer that would underlie the film.
To further explore the potential of the approach, he is working with GaN-based LEDs that feature an integrated photonic crystal region and a quantum dot film. Another avenue of study will be to investigate the recycling of guided light by deep photonic crystals.